


Fool Me Once

by hyliank8



Category: Undertale (Video Game)
Genre: Angst, Betrayal, Corrupted Gaster, Fairly descriptive rape/non-con, Gaster is Not his usual sweet self, Heavy Angst, Helplessness, Hurt No Comfort, M/M, Not incredibly explicit but definitely NSFW, Panic Attack, Power Kink, Powerlessness, Praise Kink, Sadism, Victim Blaming, W. D. Gaster is not related to Skelebros, also there's dick this time sorry :///, past abusive relationship, past trauma
Language: English
Status: Completed
Published: 2020-03-19
Updated: 2020-03-19
Packaged: 2021-02-28 04:13:32
Rating: Mature
Warnings: Rape/Non-Con
Chapters: 1
Words: 2,063
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22843840
Author URL: https://archiveofourown.org/users/hyliank8/pseuds/hyliank8
Summary: There’s a reason Sans doesn’t get close to people.  Gaster takes this as a personal challenge in the worst way.(Please mind the tags and warning.)(Русский перевод доступен в заметках автора. || Russian translation is available in the author's notes.)
Relationships: W. D. Gaster/Sans
Comments: 9
Kudos: 71





	Fool Me Once

**Author's Note:**

> uhhhh HI i'm thirsty for unhealthy fucked-up sanster as always sO THIS IS A VERY NOT NICE FIC andd i am just going 2 Pretend like i am not Extremely Self-Conscious abt posting this,,
> 
> >throws this up real quick b4 i change my mind  
> >frantically sprints in the opposite direction
> 
> Update: Translation into Русский/Russian [available here](https://ficbook.net/readfic/9705178) thanks to [Павлин](https://ficbook.net/authors/2914159) :)

There’s a reason Sans doesn’t get close to people.

A distant part of Gaster’s mind rings with a memory. It’s from a part of his mind buried behind what feels like years of time, but in reality—well, his DT experiments weren’t that long ago. It was from before the experiments, before he’d tested the substance on himself in the privacy of his personal lab—before he’d had a taste of that raw _power_.

Sans hadn’t wanted to talk about his past relationship. The one that had left him with nightmares, that had made him flinch every time someone brushed by him at the labs. But after months of working with Gaster, of them growing closer, Sans had told him everything. Sans trusted him.

A grin spreads across Gaster’s face. That makes things easier for him now.

Sans’ relationship had left him with a cracked soul and desperate need to be loved; the latter of which Gaster can tell, no matter how much Sans would likely want to deny it. It’s etched into the smallest moments that Gaster observes—the overwide grin that stretches across Sans’ face when someone laughs at a pun he makes, in the gentle blush that settles over his face when Gaster works across from him.

The desperate way he clings to Gaster in the rare hug he’d receive, the desperate way he clings to every sparse intimate moment they share like he never wants to let go.

But too much all at once would only push Sans away—so Gaster starts small. With small, delicate touches, building Sans up to it, getting him used to Gaster’s close presence throughout the day. Sans stops flinching eventually and even—even, it seems, begins to search out the minute moments of contact. Almost leaning into the touch, when Gaster rests a hand on his shoulder, and Gaster wonders how long it has been since Sans has felt comfortable enough around another monster to be touched without warning like this.

Gaster’s soul aches in anticipation.

When Gaster finally gets Sans alone in his office—and it doesn’t take much—Sans is delightfully receptive to Gaster’s wandering hands, to his heated kisses. 

He’s so delightfully lost in the sensation, it takes him an extra beat to react to Gaster taking things further. Gaster can tell it’s starting to become too much for him—Sans for the first time that day is not leaning into the touches, but rather shying away from them. Gaster’s wandering hands—he’s called up a couple pairs at this point—are on him everywhere, slipping under his shirt as they kiss—touching, feeling, documenting every inch of his bones, itching with the desire to take, to _claim_.

“g-gaster, wait—“ Sans gasps when a conjured hand slips underneath the waistline of his shorts. 

“Shh shh,” Gaster places a finger over his mouth, and Sans is drunk on his touch. “You know I’d never hurt you, Sans,” he says, and he hopes to the stars that the giddiness isn’t evident in his voice. “Not like _he_ did, never. You know that.”

Sans is visibly taken aback by the mention of the monster who harmed his soul. “i…” Sans trails off, unable to form a coherent thought. 

“You don’t need to talk.” Gaster says. 

Sans’ eyelights are hazy. He’s such a mess of overwhelmed feeling that he barely seems to notice his shorts come off, doesn’t notice until they’re on the floor and he’s completely bare before Gaster’s hungry gaze.

But his mind must still be clouded, and it’s perfect, he doesn’t realize what’s going on, doesn’t realize what's happening below until—

Sans cries out suddenly when Gaster pushes inside of him, and there’s a sharp twinge at the edge of the sound that doesn’t come from any cry of pleasure. Gaster feels the wave of heat inside of him rise further—he swears that cry is the most magnificent sound he’s ever heard. 

“g-gaster, f-fuck, wait—” Sans’ voice is panicked and taut with pain already, oh stars, it’s even better than Gaster ever could have dreamed, and he’s so deliciously tight too—

~~_stop, stop,_ a voice in his head is screaming, a voice in his head buried so deeply in his mind he can hardly hear it _. stop, you’re hurting him, you love him and you’re_ hurting ~~ _~~him!~~ _

Gaster pulls out and pushes all the way back in before ~~the voice~~ Sans can protest.

“ _‘dings—!_ ” Sans hands snap up to push against Gaster’s broad chest, trying with everything he’s got to push him off, to make the pain stop. His strength is so pathetic, Gaster marvels. Such weak, fragile magic, such a pitifully weak soul… like he was _made_ to be overpowered. Gaster groans at the thought. He wants Sans all to himself—he wants to take him, claim him as his own, and never, _never_ let him go.

Sans is visibly confused by Gaster continuing despite his protests and overwhelmed tears are forming in his eyes. “ ‘d-dings, why are you—” Gaster cuts him off with another rough thrust. “ _ah—!_ gaster, _stop!_ g-gaster—!”

“Mm, you feel so good around me, you know that?” Gaster sighs as though Sans has said nothing. “Such a good boy…”

“i… i’m… ‘dings, you’re—” Sans lets out a wet sob, and _fuck_ , he tightens around Gaster so _wonderfully_ when he’s upset like that. “you’re hurting me, it—” Sans makes another feeble attempt at pushing Gaster off when he pulls out for another thrust, but to no avail—he cries out when Gaster forces himself into him again. 

“it’s t-too much, it hurts so bad, please, _please—_ i’m s-sorry, pl-please, st-stop, i c-can’t do this, i c-c- _can’t—!_ ” Sans pounds his hands uselessly against Gaster’s chest. He sounds so broken already, it’s _amazing—_ it’s as though he can barely form words, his phrases verging on incoherency as his tone pitches into hysteria. Gaster grins as he gazes into Sans’ hazy, tearful eyelights, hoping that if he admires Sans’ horrified expression long enough it will be burned into his memory.

He wants to have a taste of Sans’ fear, to _feel_ it, so he reaches out with his magic to sense for Sans’ soul. He finds that the fragile, cracked thing is trembling in terror, struggling to deal with the unwanted sensations bombarding its owner. And _stars_ , the distress is such a _delicious_ feeling; he groans as it echoes within his own soul. He can feel Sans’ agonized confusion too, can tell Sans is trying to justify this to himself by telling himself that Gaster must not understand how much he’s hurting him—that he must be continuing with this because he just doesn’t understand, right? But as the moments pass, Gaster can feel the excuses crumbling away and Sans’ self-doubt taking its place—and hears Sans’ small, frail bones rattling as his body begins to shake uncontrollably.

“w-why,” Sans says, though it hardly seems as though he’s addressing Gaster. His voice is shaking as his palms continue to push hopelessly against Gaster’s chest. “i-i don’t understand, i thought-” 

_Oh, how adorable_ , Gaster marvels.

“What did you think, Sans?” Gaster taunts. “That I liked you? Loved you, even? That you were worth more than this? If you didn’t want this, you shouldn’t have made yourself so damn _tempting_.”

“i—” Sans can’t form his thoughts into words anymore, it seems. Or perhaps his trauma-addled brain won’t form the thoughts in the first place—Gaster’s not sure which theory he likes more. “n-no, i…” He breaks off with a gasp with Gaster’s next thrust.

As Gaster works up a pace pushing into the helpless monster beneath him over and over, Sans’ protests taper into wordless, pathetic sobs. Sans continues pushing feebly against Gaster’s chest, tears streaking down his cheekbones, and Gaster notes with admiration how beautifully the tear tracks catch the dull light in his office. Sans looks absolutely _perfect_ like this—it’s so clear that he was _made_ for this, made to be ruined, made for Gaster and Gaster _only_ to use for as long as he wants, however he wants.

Sans’ pitiful sobs only spur Gaster on, and eventually Sans gives up trying to push him away. He resigns himself to covering his eyes with his forearm so that he doesn’t have to look Gaster in the eyes, his face flushed with shame. His breath is coming in short, labored wheezes, and it seems that he can’t stop his body’s shaking. Gaster wonders idly if Sans is having one of his silent panic attacks.

~~_stop this, you know what his attacks are like, you know when he has one he can’t breathe and he feels like he’s going to die, you’ve comforted him through so many, stop this, stop this_ now _!_~~

He did this, _Gaster_ caused this; he’s so powerful now compared to Sans that Sans has no choice to submit to Gaster’s will no matter how much it hurts him. And Gaster loves it, _loves_ that Sans is perfectly vulnerable beneath him, completely at Gaster’s mercy. Gaster could do whatever he wanted, he knows, _anything_ he wanted to him right now and Sans would be forced to take it, would be absolutely _helpless_ to stop him, and the thought makes the hot pleasure in his core coil ever tighter.

Save for his trembling, Sans is hardly moving anymore. When Gaster occasionally gives a particularly harsh thrust, he whimpers miserably and his body tenses up in pain, but he doesn’t protest. His words have left him, it seems, and even his distressed wheezing has slowed. Gaster senses Sans’ soul beating weakly beneath him, struggling to deal with the pain and the betrayal from one of the only people Sans ever managed to get close to.

When Gaster’s peak crashes over him, he holds the quivering mass of bones beneath him close to his chest as his entire body shudders. Sans’ body heaves with sobs anew, more tears leaking from his eye sockets as Gaster rides out the wave of pleasure coursing through his soul. It is, without a doubt, the most wonderful orgasm Gaster has ever had.

He pulls out of the pitiful skeleton beneath him and steps back with a satisfied sigh, pulling a single tissue from the box on his desk. As he idly wipes himself clean, he takes a moment to appreciate what a sweet mess he’s made of his assistant. Sans trembles on the desk, his teeth clenched tightly together. His soul’s shivering is so intense that Gaster can sense it from a distance, without having to reach out with his magic to feel for it. Sans’ arm is still thrown over his face, and Gaster can feel his fear—Sans is terrified to look Gaster in the eyes. 

Gaster’s release glistens beautifully on Sans’ thin bones alongside his sweat, only visible under Sans’ hastily hiked-up t-shirt beneath his lab coat. Gaster runs a gentle hand over the sensitive bones of Sans’ pelvis, appreciating how they’ve been dampened with Sans’ terrified sweat. The touch is light, but Sans flinches away as though he’s been burned, a choked, terrified sound bubbling up in his throat. 

Gaster chuckles darkly and takes his hand away. _He’s mine now_ , Gaster reflects with satisfaction. _Mine to do with as I please… my toy, my plaything._ Heat rises up again within him at the thought, but he doesn’t let himself get carried away with feeling. 

There will be plenty of time to further indulge himself later.

Gaster fixes his own clothing, buttoning up his pants and smoothing down his shirt, making sure his lab coat is properly in place. “You did an extraordinary job, my assistant,” Gaster croons. “You may want to consider cleaning up before heading back home to that brother of yours, hmm?” At the mention of his brother, Sans lets out another wordless, anguished sob, and Gaster lets a content smirk settle on his face at Sans’ distressed shame.

“I’ll see you tomorrow morning,” Gaster says. He walks to the door, pausing thoughtfully before he makes his exit. “...and, meet me in my office after work again, won’t you, Sans?” _Unless you want to lose your only means of supporting your family_ is left unspoken; Gaster knows Sans will quickly come to understand his position.

Sans chokes up, and Gaster steps out of the door, leaving his assistant alone to clean up and pull himself back together.

**Author's Note:**

> i wrote the majority of this while plastered (tho tbh it was more like autocorrect was writing it for me) so i had to go back and change a lot of tenses, BU T honestly it took me so long to finish this sober even tho there was Very Little Actual Writing left to dooooo it was just. interestingly difficult 2 be on the same wavelength as my drunk self!! (who is apparently very angsty, hehe,,) the style feels a lot different from what i'm used to writing which i think is why it was so hard to finish this while sober >.< ngl i'm not super happy with the quality of this rn and there's a decent chance i'm going to delete this later bc i've never posted noncon b4 and i'm chickenshit sldkfj but i wanna get this out there anyways for right now so!!! here!!!
> 
> come follow/chat w/ me on [tumblr](https://hyliantimelordin221b.tumblr.com) or [ twitter(nsfw)!!! ](https://twitter.com/K8Maybe) i mainly talk about sanster a whole lot, come join me!! ^^
> 
> anyways!!! pls feel free 2 lemme know what u thought!!!


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